Motional feedback amplifier

ABSTRACT

A motional feedback amplifier, having an impedance bridge, one arm of which comprises a loudspeaker and which is balanced at the frequency of the loudspeaker with the diaphragm blocked, comprising a low-pass filter in the feedback loop between the reading terminals of the bridge and the input terminal for the feedback voltage to the amplifier and the characteristic of which is adjusted so as substantially to suppress the feedback voltage in the frequency zone in which the false readings appear and above that zone, a low-frequency corrector in the conventional negative feedback loop, the said filter being preceded by a transistor, two control electrodes of which, are fed with a signal from the reading terminals of the bridge and the output electrode of which delivers the motional feedback signal to the amplifier.

United States Patent Korn 1 Mar. 7, 1972 [54] MOTIONAL FEEDBACKAMPLIFIER [72] Inventor: Tadeusz Ken, 58 rue Mercelis, Brussels, ""9""Exami'.wr Kathleen H'Claffy Belgium 1 Assistant Exammer-Horst F. BraunerAttorney-Waters, Roditi, Schwartz & Nissen [22] Filed: Aug. 28, 1969 211Appl. No.: 853,665 [571 ABSTRACT A motional feedback amplifier, havingan impedance bridge, [30] Foreign Application Priority new one arm ofwhich comprises a loudspeaker and which is balanced at the frequency ofthe loudspeaker with the Aug. 29, 1968 Belgium ..720-1 15 diaphragmblocked, comprising a low-pass filt i h f d. back loop between thereading terminals of the bridge and the [52] U.S. Cl ..l79/1 F inputterminal f the f db k lt to the lifi and [51] f 3/00 the characteristicof which is adjusted so as substantially to [58] Field of Search ..I79/1 F suppress the feedback voltage in the frequency zone in which thefalse readings appear and above that zone, a low-frequen- [56] Reerencescued cy corrector in the conventional negative feedback loop, the

UNITED STATES PATENTS said filter being preceded by a transistor, twocontrol electrodes of which, are fed with a signal from the reading ter-2,887,532 5/1959 wpmel' 1 79/1 i l fth b idge and the output electrodeof which delivers g g the motional feedback signal to the amplifier. erier..... 3,530,244 9/1970 Reiffin ..179/l F 2 Claims, 6 Drawing Figures4F C01EGTOE I 1 1 A ---?MflL/F/Ee a F l A 120 F71. r52

A I J I 19 l 7 1 8 TEAMS/STOP Patented March 7; 1972 3,647,969

5 Sheets-Sheet 1 4/14/ 4 lF/EE Patentd March 1, 1972 3,647,969

3 Sheets-Sheet 2 FIGA Patented March 7, 1972 3 Sheets-Sheet 5 I7 r I I II I l I I I I I I I I I I I 0 .l U Q a FIGLE FIG.6

.. I i l 1 I I 24 MOTIONAL FEEDBACK AMPLIFIER This invention relates tothe construction of a motional feedback amplifier whereby the mechanicalresonance of the loudspeaker to which it is connected can be suppressed,said motional feedback being generated by an impedance bridge, one armof which consists of a loudspeaker and which is balanced when theloudspeakers diaphragm is blocked.

The invention relates to the construction of means adapted to make suchamplifiers reliable and suitable for mass production.

Referring to the accompanying drawings:

FIGS. 1 and 2 show a prior art motionalfeedback amplifier;

FIG. 3 is a frequency-reactance graph of the amplifier of FIGS. 1 and 2;

FIGS. 4 to 6 illustrate a motional feedback amplifier in accordance withthe present invention.

A motional feedback amplification method is already known of the typeshown in FIG. 1 of the accompanying drawings, being based on thefeedback principle comprising a bridge 2 with one arm formed by theloudspeaker 3, the said' bridge being prebalanced for the value of thisarm equal to the electrical impedance of the coil of the loudspeakerwith the diaphragm at rest. Thus the electromotive force induced in thecoil by its vibrations in other words the appearance of the motionalimpedance of the loudspeaker-put the bridge off balanceand produces areading voltage between the terminals 8 and 10. This voltage, which isassumed to be proportional to the velocity of the diaphragm, is appliedas a negative feedback to the input of the amplifier 1 (terminal 8), inorder to stabilize the velocity, i.e., suppress the mechanical resonanceof the loudspeaker.

Unfortunately, the operation of the bridge 2 is not as simple as wasassumed, and the circuit shown in FIG. 1 gives results which are hardlyreproducible and frequently less satisfactory than without motionalfeedback at all.

Various known proposals have been published and even patented in orderto improve this method, but without giving an industrially applicableresult.

Thus despite its great theoretical advantages, it has never beenpossible to put this method into practice reproducibly.

The reason for these failures hitherto is that ithas not been possibleto find the true cause of malfunctioning of the bridge 2. In order toexplain the foregoing, reference will be made to FIG. 2, whichillustrates the physical components forming the bridge 2.

It will be obvious that if theelectrical impedance of the loudspeakerwith the diaphragm blocked 11 were composed solely of a pure resistance12, the bridge 2 would readily operate in'accordance with the elementaryprinciple set forth hereinbefore. However, the electrical impedance ofthe coil of the loudspeaker 11 includes an inductive component 13 whichseriously obstructs the required operation of the bridge 2. The authorsof known proposals had been aware of the existence of the element l3 butthey have erroneously interpreted its obstructive action, i.e., theyhave considered it solely as a difficulty in correct prebalancing of thebridge 2 throughout the audio-frequency range (with the diaphragmblocked). As a remedy, they have proposed that one of the other arms ofthe bridge 2 should contain adequate'reactive elements, e.g., aninductance in series in the arm 4 or 5, or a capacitor in parallel witha resistor in the arm 6.

To obtain strict prebalancing of the bridge 2 throughout the wholeaudio-frequency range, some authors have proposed that the reactiveelements used should be highly complex with a characteristic allowingfor taking into consideration the resistive losses of the inductance 13in the high-frequency zone.

To obviate these complications in prebalancing the bridge 2, someauthors have proposed to attenuate the operation of the entire system inthe high-frequency zone by means of a capacitor disposed, for example,between the reading terminals of the bridge 8 and 10. This solution wasalso supposed to attenuate the effect of the partial vibrations of thediaphragm appearing in the same high-frequency zone and which have alsobeen considered as one of the main sources of difficulty.

Hereinafter it will be shown that a rudimentary solution of this typehas not been able to eliminate the true cause of difficulty. It shouldalso be noted that the provision of a capacitor directly at the readingterminals is contrary to the elementary theory of measuring bridges.

The present invention is based on a careful investigation of the circuitshown in FIG. 2.

This investigation shows that the true obstructive action is due to thecombination of the positive reactance 13 with the reactive component ofthe motional impedance 14, which becomes negative above the resonantfrequency of the loudspeaker.

FIG. 3 is a frequency-reactance graph showing this concept.

Referring to this graph, the line a denotes the reactance of 13; thecurve b denotes the reactance of 14; the curve 0 denotes the resistanceof 14.

The line f 0 relates to the frequency corresponding to the resonance ofthe loudspeaker where the motional feedback should be the mosteffective.

This Figure shows that the total reactance of the arm 3 is cancelled outat the frequencies f and f at which the moduli of said two reactances ofopposite sign become equal (X 14= Thus the bridge 2 which was strictlyprebalanced with the diaphragm blocked gives readings at frequencies fand j, which are in no case proportional to the velocity of thediaphragm and which seriously disturb the response curve of the completesystem. These readings will hereinafter be referred to as false readingsof the motional impedance by the bridge 2.

This behavior of the bridge 2 is somewhat similar to the operation ofthe known bass reflex loudspeaker but produces another which is stillmore disturbing since it is situated at a higher frequency. The amountof disturbance by the false readings of the bridge 2 at f, and f dependson the value of the series resistive component R 14 (curve c) of themotional impedance 14 which appears at these frequencies. This value ishighly uncertain since it depends on the mechanical and acoustic loadsof the loudspeaker, which are frequently uncontrollable. That explainswhy the known constructions gave unstable and nonreproducible results.

It is very important to note that the frequency zone at which the falsereadings of the bridge 2 appear is very close to the operating frequencyof the system f,,. The problem is therefore physically and technicallyquite different from the known problems and more particularlyprebalancing of the bridge or partial vibrations of the diaphragm whichappear only in the much more remote frequency zone. Consequently thesolutions forming the subject of this invention must be distinctlydifferent from all those known in the technology.

The amplifier according to this invention comprises an inseparable groupof means which provide an efficient motional feedback action (at least10 d.) at the resonant frequency f of the loudspeaker but whichsufficiently suppresses the reading action of the bridge 2 in thefrequency zone at which the false readings appear and which is veryclose to the said frequency f,,. This group of means comprises thefollowing elements (FIG. 4);

l. A transistor 18 for reading of the bridge 2, its input electrodes19-20 being connected, according to the polarity of the required signal,respectively to the terminal 8 and the terminal 10 of the bridge 2, andthe output electrode of which is shown at 21. The object of the reading"transistor is not only to bring the reading signal into reference withrespect to the ground, but also amplify it and to form a high-impedancesource for the circuits following one another in the motional feedbackloop.

2. The reading transistor is followed by a filter I7 comprising twocapacitors 25 and 26 (FIG. 5) connected between the signal line andground. To prevent the transfer function of the filter from assumingexcessive values at very low frequencies, which may result in adisturbing positive feedback, the filter also comprises a capacitor 27in series in the signal line which attenuates the signal at the saidvery low frequencies. The said series capacitor also corrects thephase-shift of the filter in the effective frequency zone. However, thesaid filter with two capacitors to ground, in other words with two RCnetworks, does not yet have sufficient slope to select the effectivefrequency zone in relation to the disturbing zone. Unfortunately, anyincrease in the number of RC networks results in a disturbingphase-shift which interferes with the operation of the system. To obtainadequate discrimination of the effective zone in relation to thedisturbing zone without disturbing phase-shift, the amplifier has athird element which forms part of the group of means proposed in theinvention and which is:

3. A low-frequency corrector 24 (FIGS. 4 and 6) disposed in theconventional negative feedback loop with which the amplifier accordingto this invention must be provided and which corrector consists, forexample, of the resistor 29 and capacitor 28 in parallel. The object ofthis corrector is to increase the amplifier gain at low frequencies,thus reinforcing discrimination between the effective zone and thedisturbing zone of the motional feedback, because the said amplifier issituated in the loop of the latter.

What is claimed is:

1. A motional feedback amplifier comprising an impedance bridge forproviding said feedback, one arm of said bridge including a loudspeaker,said bridge being balanced with a loudspeaker diaphragm blocked at theresonance frequency of the loudspeaker, a first feedback loop having alow-pass filter provided therein extending between reading terminals ofthe bridge and an amplifier input terminal for reading the feedbackvoltage to the amplifier, said amplifier being adapted to substantiallysuppress the feedback voltage in and above the frequency zone in whichany false readings of the bridge appear, and a second feedback loophaving a low-frequency corrector provided therein constituting anegative feedback loop of said amplifier, an active transistor elementin said first feedback loop preceding said low-pass filter, said elementincluding two control electrodes which form an emitter and a base of theelement and which are supplied with a signal from the reading tenninalsof the bridge, and an output electrode of the element adapted to deliverthe feedback signal to the filter and to the amplifier.

2. An amplifier as claimed in claim 1, wherein said filter comprises atleast two capacitors connected between a signal line and ground, and acapacitor connected in series in the signal line which attenuates thegenerated signal at very low frequencies and reduces the phase-shift inthe operating zone of frequencies.

1. A motional feedback amplifier comprising an impedance bridge forproviding said feedback, one arm of said bridge including a loudspeaker,said bridge being balanced with a loudspeaker diaphragm blocked at theresonance frequency of the loudspeaker, a first feedback loop having alow-pass filter provided therein extending between reading terminals ofthe bridge and an amplifier input terminal for reading the feedbackvoltage to the amplifier, said amplifier being adapted to substantiallysuppress the feedback voltage in and above the frequency zone in whichany false readings of the bridge appear, and a second feedback loophaving a low-frequency corrector provided therein constituting anegative feedback loop of said amplifier, an active transistor elementin said first feedback loop preceding said low-pass filter, said elementincluding two control electrodes which form an emitter and a base of theelement and which are supplied with a signal froM the reading terminalsof the bridge, and an output electrode of the element adapted to deliverthe feedback signal to the filter and to the amplifier.
 2. An amplifieras claimed in claim 1, wherein said filter comprises at least twocapacitors connected between a signal line and ground, and a capacitorconnected in series in the signal line which attenuates the generatedsignal at very low frequencies and reduces the phase-shift in theoperating zone of frequencies.